is

class A

{

     public virtual void F1() {

          Console.WriteLine("A");

     }

}

class B : A

{

     public override void F1() {

          Console.WriteLine("B");

     }

}

class C

{

}

static void Main(string[] args)

{

     A obj = new B();

     if (obj is A)

     {

          Console.WriteLine("obj is A");

     }

     if (obj is B)

     {

          Console.WriteLine("obj is B");

     }

     if (obj is C)

     {

          Console.WriteLine("obj is C");

     }

     else {

          Console.WriteLine("obj is not C");

     }

     Console.ReadKey();

}

obj is A

obj is B

obj is not C